1. Field of the Invention
The present invention relates to a monolithic silicone that has continuous through pores, is flexible, requires no support and is useful for separation, purification and concentration of a sample or the like, and a treatment method for a sample or the like including separation, purification or concentration.
2. Description of the Related Art
Packing materials and capturing agents utilized in separation, purification and concentration of samples, analytes and the like are broadly classified into three types: packing materials in which stationary liquids such as silicone are immobilized on supports such as a silica gel and diatomite; adsorbent packing materials using activated carbon and zeolite; and polymer packing materials such as phenol resins, acrylate resins and styrene-divinylbenzene copolymers.
A mechanism of separating and concentrating an analyte from a gas phase such as the air by using a stationary liquid type packing material is regarded as gas-liquid partition equilibrium, and a mechanism of separating and concentrating an analyte from an aqueous sample by using this type of packing material is regarded as liquid-liquid partition equilibrium. The corresponding mechanisms working in using an adsorbent packing material are regarded respectively as gas-solid equilibrium (i.e., adsorption equilibrium between a gas phase and a solid phase) and liquid-solid equilibrium. When a polymer packing material is used, an analyte may be adsorbed by the polymer packing material, and it is regarded that mechanisms of both gas-solid equilibrium and gas-liquid equilibrium or both liquid-solid equilibrium and liquid-liquid equilibrium work in this case, but retention in pores of a porous polymer with van der Waals force is dominant in this mechanism and the gas-solid or liquid-solid adsorption equilibrium strongly works.
An adsorbent packing material and a polymer packing material have high mechanical strength in general, and an adsorbent or polymer packing material in a particulate form or recently monolithic form is used to be filled in a column tube. Since spaces between the particles or through pores of the monolith function as passages for a gas or a liquid in such a packing material, there is no need to use a support.
Besides, an absorbent packing material and a polymer packing material have strong retention force and may efficiently retain a volatile substance but are not suitably used for separation, purification and concentration of an unstable substance. Particularly, such a packing material needs large energy for desorbing a substance and many chemical species are denatured at the time of desorption.
Examples of a packing material not using a support are polymer packing materials. The polymer packing materials include phenol packing materials, styrene-divinylbenzene packing materials and the like, and packing materials named as Tenax TA and Porapack (registered trademark) are commercially available respectively as a phenol packing material and a styrene-divinylbenzene packing material. These packing materials are in the form of particles. Other examples of the packing material not using a support are organic polymer monoliths (see “Bunseki Kagaku” vol. 57, No. 7, 517 (2008)). Organic polymer monoliths of acrylamide base, methacrylic ester base and styrene-divinyilbenzene base have been mainly studied and examined, and organic polymer monoliths of styrene base and methacrylic ester base are commercially available.
Such a polymer packing material conducts adsorption (concentration) by capturing, with van der Waals force, target chemical species within pores present on the surface of the polymer. For desorbing the adsorbed chemical species, a method of heating the polymer packing material and purging the chemical species with an inert gas (which method is designated as thermal desorption) is employed in GC analysis. When Tenax TA is used in this manner, dehydration of 2-methylisoborneol may be caused. This is a problem occurring because chemical species to be analyzed (hereinafter referred to as analytes only in the case of chemical species to be analyzed) enter (are adsorbed into) pores present on the surface of the polymer packing material.
When analytes adsorbed by a packing material are to be desorbed with a solvent, the packing material is packed in a vessel for use. Therefore, a space designated as a “bed volume” is formed among particles of the packing material, and hence, a solvent in an amount not only for desorbing the analytes but also for filling the space is excessively required in desorbing the solvent. Since an organic solvent is used also for replacing the bed volume, the concentrated analytes are disadvantageously diluted.
On the other hand, a partition equilibrium type packing material may easily dissolve and desorb analytes in and from a stationary phase and is suitably used for measuring unstable analytes, but it has so weak retention force that a large amount of packing material is necessary for extracting objective analytes from a large amount of matrix. In particular, silicone coated as a stationary liquid for a partition equilibrium type packing material is most frequently used because it is an excellent material stable over a wide temperature range from a low temperature to a high temperature.
A silicone polymer has a property to dissolve analytes like a liquid and may work as a packing material (a stationary liquid) on the basis of a principle of partition equilibrium. Concentration or desorption of analytes on the basis of this principle is free from the problem occurring in a conventional polymer packing material in concentration or desorption of analytes within pores.
In general, polydimethylsiloxane (hereinafter abbreviated as “PDMS”) foam is formed into a porous material by mixing a foaming agent with raw materials for forming pores by a gas produced through a chemical reaction. Therefore, the PDMS foam has an ununiform porous structure and pores formed therein are closed, and hence it is impossible to construct a passage penetrating through the structure.
Since the PDMS foam does not have a passage within the structure thereof and does not have a uniform skeleton, it takes time for material transfer between phases (i.e., between a gas phase or a liquid phase and silicone) of analytes, and hence, the PDMS foam is presumed to be poor in extraction efficiency and desorption efficiency.
When analytes concentrated in the PDMS foam are subjected to the thermal desorption to be introduced to gas chromatograph, it also takes time for material transfer. Therefore, a band is spread in introducing a peak sample for the analysis, and it is indispensable to focus a peak band. If a peak band is neglected to focus, a peak shape of a resultant chromatogram is deteriorated, which may lower accuracy in the analysis.
An example of analysis of a food flavor through recovery with a PDMS film has been reported by Dong-Sun Lee (see “Bull. Korean Chem. Soc, 2011, vol. 32, No. 10, 3603). According to this report, a film-shaped PDMS obtained by mixing PDMS in a liquid form and a curing agent is cut into a disc shape to be used as a solid phase extracting material.
The thus produced PDMS film is put in a sample bottle together with a sample and the sample bottle is tightly stoppered. A volatile component volatilized from the sample is solid-phase extracted onto the PDMS film through the partition equilibrium. Thereafter, the PDMS film is taken out, acetonitrile is charged in a microtube as a desorption solvent, and the sampled PDMS film is introduced thereto. The microtube is tightly stoppered, and after stirring for 3 seconds, the PDMS film is taken out of the microtube. One μL of the thus obtained acetonitrile solution is introduced into a GCMS for analysis. Although a recovery factor for the analytes may be increased by increasing the area of the film, it also takes time both for reaching the partition equilibrium and for performing desorption because the PDMS film does not have a passage therein similarly to the PDMS foam.
SPME (Solid Phase Micro Extraction) using a glass needle coated with a silicone polymer (such as PDMS) is known and is commercially available (see http://ir.lib.hiroshimaru.ac.jp/metadb/up/diss/disc_ko504 3.pdf). The amount of silicone used in the SPME is approximately 0.2 mg. This method has a demerit that the amount of silicone polymer used as a stationary liquid is too small to perform highly sensitive analysis. Furthermore, it also takes time both for reaching the partition equilibrium and for performing desorption because the SPME does not have a passage therein similarly to the PDMS foam.
SBSE (Stir Bar Sorptive Extraction) using a magnetic stirrer coated with a silicone polymer (PDMS) is known and is commercially available (see http://ir.lib.hiroshimaru.ac.jp/metadb/up/diss/disc_ko504 3.pdf). The amount of silicone used in this method is several tens times as large as that used in the SPME (specifically, 24 mg). Since the amount of liquid phase is larger than in the SPME, a recovery factor may be remarkably increased. This method has a demerit that the amount of silicone polymer used as a stationary liquid is so large that it takes time for desorbing analytes sorbed through the partition equilibrium. Therefore, when the analytes are to be analyzed by thermal extraction in the GC, it is necessary to perform a band focusing operation using liquid nitrogen or the like, and therefore, productivity in the analysis is degraded.
In this manner, packing materials and capturing agents conventionally used for separation, purification and concentration of samples have problems summarized as follows. The adsorbent packing materials and the polymer packing materials generally have strong retention force and may efficiently retain volatile substances but are not suitably used for separation, purification and concentration of unstable substances. In using these packing materials, energy is necessary particularly for desorption, and many of chemical species are denatured in the desorption.
The partition equilibrium type packing materials, particularly silicone, may not secure passages for a gas or a liquid owing to their properties unless they are immobilized on some support for use. Furthermore, on the basis of a theoretical formula of the partition equilibrium, although the recovery factor from a sample may be improved when a large amount of stationary liquid is used, the amount of packing material that may be immobilized on a support is limited.
When the conventional silicone polymer is used, since it takes time for desorption of analytes, the productivity of the analysis is lowered in the thermal extraction due to a band focusing operation and the like.